Stephen N. Kukureka

The effect of fibre reinforcement on the friction and wear of polyamide 66 under dry rolling–sliding contact

Abstract An investigation into the effect of fibre reinforcement on the friction and wear of PA66 in rolling–sliding contact is reported. Three types of short fibre—aramid, carbon and glass—were examined with the composites running against identical materials in a twin disc machine. It was found that the aramid-fibre reinforcement did not significantly alter the friction of the matrix material. However, both the carbon-fibre and glass-fibre reinforcement reduced the coefficient of friction substantially. Wear of the aramid- and carbon-fibre composites was essentially linear with time and generally around ten times greater than that of the unreinforced material. The wear of the glass-fibre composite was complex with an initial period where the wear rate was similar to that of the unreinforced material. After a significant depth of wear had occurred the wear rate changed to a value similar to, but slightly higher than, that of the other reinforced materials. It appears that one of the major benefits of introducing fibre reinforcement, particularly glass, is that it reduces the coefficient of friction and hence allows the material to be used for higher duties without exceeding the softening point of the matrix. This increase in duty is, however, at the expense of an increased wear rate and shorter component life.

Accelerated aging for testing polymeric biomaterials and medical devices

Elevated temperature is frequently used to accelerate the aging process in polymers that are associated with medical devices and other applications. A common approach is to assume that the rate of aging is increased by a factor of 2(DeltaT/10), where DeltaT is the temperature increase. This result is a mathematical expression of the empirical observation that increasing the temperature by about 10 degrees C roughly doubles the rate of many polymer reactions. It is equivalent to assuming that the aging process is a first order chemical reaction with an activation energy of 10R/log(e)2, where R is the universal gas constant. A better approach would be to determine the activation energy for the process being considered but this is not always practicable. The simple approach does not depend on the temperature increase, provided that it is not so great that it initiates any physical or chemical process that is unlikely to be involved in normal aging. If a temperature increment theta were to increase a given polymer reaction rate n times, then an elevated temperature would increase the rate of aging by a factor of n(DeltaT/theta).

A study of the tribological behaviour of polyamide 66 with varying injection-moulding parameters

Abstract Injection-moulded polyamide 66 has been widely used for gear and bearing applications but the mechanisms of wear in such components are not fully understood. In particular, little attention has been paid to the effects of processing on materials microstructure and tribological performance. Polyamide 66 discs were moulded using a range of mould wall temperatures. The discs in the as-moulded and machined conditions, representing a variety of microstructures and surface conditions, were tested (unlubricated) on a twin-disc wear testing machine, previously shown to be useful for separating the effects of rolling and sliding in non-conformal contacts. The test parameters of speed, load and slip ratio are useful in simulating wear and friction in applications such as gears where the geometry and complex mechanics mask the fundamental materials behaviour. Optical microscopy, between crossed polars, and differential scanning calorimetry were used to determine bulk crystallinity and spherulite morphology as a function of process parameters. The as-moulded (transcrystalline) and fine-machined surfaces were characterised using optical examination and scanning electron microscopy. The different bulk and surface structures were related to wear performance when running against similar and dissimilar (steel) materials.

Wear behaviour of acetal gear pairs

Abstract The wear behaviour of polymer gear pairs has been little studied. Most wear studies have used pin-on-disc or polymer against steel tests. The non-conformal contact and high transient temperatures in polymer gear pairs make it essential to study wear under actual running conditions. This paper describes the design and operation of a novel test rig for measuring the wear behaviour of polymer gears continuously. Preliminary results are presented and comparisons made between failures in acetal and nylon. Wear of acetal gears is studied in more detail. It is shown that wear of acetal gears is a complex process. Optical and scanning electron microscopy (SEM) show the driving and driven gears to have different features on the worn flanks, with a ridge forming at the pitch line on the driven gear and a valley on the driving gear. The wear surfaces over the addendum and dedendum are also strikingly different. This is related to the direction of rolling and friction forces on each gear face. The limits to the use of acetal as a gear material are considered. At low torques, life is limited by wear and at high loads the maximum permissible surface temperature is a limiting factor.

The wear mechanisms of acetal in unlubricated rolling-sliding contact

The wear mechanisms of acetal (polyoxymethylene or POM) running against acetal in non-conformal, unlubricated rolling-sliding contacts have been investigated over a range of loads, rolling speeds and slip ratios. A wear and friction testing machine designed and built at Birmingham allows acceptable contact stresses for polymers and composites to be combined with a wide range of rolling speeds and slip ratios under well-controlled conditions. Based on the observed drastic changes in the wear rate with running conditions, wear has been divided into two regions: mild and severe. In the mild wear region, the wear rates are below 10−6 μm cycle−1 and are independent of rolling speed and slip ratio and vary approximately with the peak hertzian contact stress. Scanning electron microscopy examination shows that the wear appears to be at least partly due to mechanical fatigue of the surface. In the severe wear region, wear rates may exceed 10−4 μm cycle−1 and are very sensitive to operating conditions. The surface temperature is shown to be above 110°C and wear is caused by lateral cracking of the surface and subsequent tearing. These results are compared with the wear behaviour of POM gear pairs and close correlation is shown to exist. It is shown that, provided the temperature limitation of the material is observed, POM gears and other non-conformal components should have a life of around 109 cycles.

Wear testing and measurement techniques for polymer composite gears

Abstract Injection-moulded composite gears using fibre-reinforced thermoplastics often exhibit increased performance compared with those made from unreinforced polymers. In general, materials are chosen on cost or simple performance grounds (such as, nominal flexural modulus or heat deflection temperature). Aspects of the performance of polymeric gears have been studied by a number of workers and efforts have been made to simulate the contact conditions during gear running. However, until now the wear performance of gears made from polymer matrix composites has not been studied systematically. While such materials have been studied using pin-on-disc or twin disc roll/slide wear techniques, no attempt has ever been made to directly compare the results from such studies with those from gear tests. This paper attempts to explain the comparative methods of measurement of various polymer matrix composite gear materials and to relate their performance to results obtained in contact simulation experiments by other workers. Methods of wear testing are compared including direct gear testing and disc testing, together with electronic (displacement) measurement, weight loss and direct measurement. A new method of characterising the wear of gears is presented, which relates actual contact conditions and gear tooth wear. This co-ordinate measurement technique provides wear rates as a function of roll angle, and hence load, sliding speed and slip ratio. It allows comparisons between gear tests and a simpler test configuration, the twin disc roll/slide test. The conclusions reached have interesting commercial implications for the producers and end users of polymer composite gears and for those involved in testing them.

Surface topography and wear mechanisms in polyamide 66 and its composites

A study has been made of the tribological behaviour of polyamide 66 (PA66) running against itself, in unlubricated, non-conformal and rolling-sliding contact. Tests were conducted over a wide range of loads and slip ratios using a twin-disc test rig. The wear and friction behaviour of unreinforced PA66 is dominated mainly by three major features: a critical slip ratio under a fixed load and running speed, macro-transverse cracks and a layer of film on the contact surface. Both the wear and friction properties of unreinforced PA66 can be improved considerably by filling with 20wt% PTFE, and the tribological mechanisms are changed significantly. This reinforcement prevents both the initiation and propagation of transverse cracks on the contact surfaces which occurred in the unreinforced material. It also decreases both the wear rate and the friction coefficient substantially. The 30wt% short glass-fibre reinforced PA66 also suppresses the transverse cracks from initiation on the surfaces. A thin film on the contact surfaces plays a dominant role in reducing wear and friction of the composite and in suppressing the transverse cracks. These results offer the prospect of enhanced applicability of PA66 in engineering components.

Evaluation of damping and elastic properties of composites and composite structures by the resonance technique

This paper describes the resonance technique for determining the stiffness and damping properties of a composite or composite structure. Pultruded GRP composites and optical fibre cables (multi-component structures) were investigated. The resonance frequencies (natural frequencies) of a material, or a system, are a function of its elastic properties, dimensions and mass. This concept is used to determine the stiffness of a vibrated material by the resonance technique, which applies only very low stresses through the application of acoustic energy. This makes it applicable to measure the stiffness of multi-element cables. The damping properties, in terms of Q−1 (internal friction) were determined by both a free exponential decay curve and half-peak bandwidth methods. The influence of specimen length and measurement set-up was investigated. The applicability and accuracy of the resonance technique for a composite structure were discussed. The measured elasticity of optical cables was found to be in good agreement with the derived theoretical value.

Wear and friction of nylon-glass fibre composites in non-conformal contact under combined rolling and sliding

Results from twin disc tests of glass fibre reinforced Nylon 66 running against itself are presented. It is shown that the low friction and wear rates often found under these conditions are due to the formation of a thin interfacial layer of nylon over the composite. After substantial wear the subsurface becomes saturated with glass fibre debris, the interfacial layer is abraded and the wear rate increases substantially. The persistence of the surface layer appears to be related to the crystallinity of the nylon matrix with low crystallinity materials running for hundreds of hours with virtually no wear while high crystallinity materials fail rapidly.

Swelling of medical grade silicones in liquids and calculation of their cross-link densities

Four medical grade silicones were swollen, until they reached equilibrium (i.e. constant mass) in eight liquids at 25 degrees C. The greatest swelling was obtained with n-heptane but the volume fraction, varphi, of the silicones in their swollen state was not significantly different (p<0.05) in this liquid than in cyclohexane. For each grade of silicone, varphi was plotted against delta(l), the liquid solubility parameter, for each liquid in which it was swollen. A second-order polynomial was plotted through the results; the minimum in this polynomial provided a value for the polymer solubility parameter, delta(p). The Flory polymer-liquid interaction parameter, chi, was calculated for the four best liquids, using Hildebrands solubility parameter theory. An alternative method for calculating chi, directly from swelling measurements, was shown to produce physically unreasonable results. The cross-link density, upsilon, was calculated, from varphi and chi, for each grade of silicone, using the Flory-Rehner equation. Since the values of two parameters involved in Hildebrands theory cannot be determined reliably and because the Flory-Rehner equation is an approximation, absolute values of upsilon cannot be obtained. However, the relative values of upsilon obtained were higher for the harder grades then for the softer grades and similarly, the grades with the higher Youngs modulus had higher upsilon values.